We have demonstrated that circulating phosphate is a critical determinant of normal growth plate maturation. Hypophosphatemia impairs appptosis of hypertrophic chondrocytes, both in vivo and in vitro, leading to the development of rickets in growing animals. Interestingly, while hypophosphatemic Vitamin D Receptor (VDR) null mice, mice with diet-induced hypercalcemia/ hypophosphatemia and hyp mice all develop progressive expansion of late hypertrophic chondrocytes due to impaired apoptosis of these cells, Npt2a null mice have not been reported to develop rickets. Our preliminary data demonstrate that, while deletion of this renal Na-dependent phosphate transporter leads to the same degree of hypophosphatemia as is observed in the other models, the growth plate phenotype in these mice is transient. An expansion of the late hypertrophic chondrocyte layer, accompanied by impaired apoptosis of these cells is observed at 16 days of age, whereas by 35 days of age, there is resolution of this abnormality. The studies in this proposal will address the hypothesis that a specific transport mechanism for phosphate is present in hypertrophic chondrocytes and mediates apoptosis of these cells. They will examine whether there is an intrinsic difference in phosphate transport in cultures of hypertrophic chondrocytes from hyp mice, Npt2a null mice and wildtype mice. They will also determine whether intrinsic cellular factors modify the susceptibility of chondrocytes, isolated from these animals, to phosphate-mediated apoptosis. Investigations will be performed to address the hypothesis that a difference in circulating hormone levels contributes to the difference in the growth plate phenotype of the Npt2a and hyp mice. Primary chondrocytes, isolated from these mice will be treated with PTH, 1,25-dihydroxyvitamin D and FGF23 to address the hypothesis that these agents modulate phosphate transport and/or susceptibility to phosphate-mediated apoptosis. To address the hypothesis that enhanced 1,25-dihydroxyvitamin D action is responsible for normalization of the growth plate of Npt2a null mice, 1,25-dihydroxyvitamin D action will be blocked by mating them with VDR null mice. Hyp mice will be treated with 1,25- dihydroxyvitamin D to address the hypothesis that impaired 1,25-dihydroxyvitamin D action and /or increases in PTH contribute to the development of rickets in this disorder. These investigations are expected to reconcile the difference in the growth plate phenotype observed in hypophosphatemic mouse models and to identify a role for circulating hormones in modulating the susceptibility of hypertrophic chondrocytes to phosphate-mediated apoptosis.